Kinetics dissociation

Dissociation reactions of the general form AB — A + B are monomolecular processes, in which the rate of decay of the complex is proportional to its concentration. The concentration dependence of cAB is given by the following differential equation  

Analysis of dissociation processes yields values for the rate constant fc2i, whose dimensions are (time)1. This rate constant is related to the lifetime (r) of the complex AB by the expression r=(k21) 1, and to the half-life (t1/2) by the expression h/2 - (ln2/fc21). 

Upon heating, the dissociation of adsorbed CO is complete at 390 K on Fe(l 10) [245]. The activation energy for dissociation of adsorbed molecules is 105 kJ/mole [383]. 

The dissociation kinetics of macrocyclic complexes have received considerable attention, especially during investigations of the nature of the macrocyclic effect. Before discussing the dissociation of cyclic ligand species, it is of benefit to consider some aspects of the dissociation of open-chain ligand complexes. 

Amine complexes. As has been mentioned in previous chapters, the dissociation of a macrocyclic ligand from its central metal ion can be extremely slow, even in the presence of strong acid. A classic example of this is provided by the dissociation under acid conditions of the square-planar [Ni(cyclam)]2+ species which has been estimated to have a half life of approximately 30 years (Billo, 1984). 

It is of interest to consider the Cu(n) reaction in more detail. The dissociation of this complex was performed in HC104 over the concentration range 0.19-0.58 mol dm-3. The reaction was observed to be first-order in complex and a plot of kobs versus [H+] confirmed the second-order dependence on [H+], The reaction has been described in terms of the following steps  

It was proposed that mono-protonation of [CuL]2+ to yield [Cu(HL)]2+ occurs initially at the axial nitrogen atom since this Cu-N bond is expected to be weaker as a result of Jahn-Teller distortion. With respect to this, it should be noted that species such as [Cu(HL)]3+ are commonly observed during potentiometric studies of the formation of Cu(n) polyamine complexes. From the proposed mechanism, the following rate law can be deduced  

In an extension of this study, the dissociation rates for the copper complexes of the corresponding 16- and 17-membered N5-macrocycles have been studied (Hay, Bembi, McLaren Moodie, 1984). Similar rate laws to that just given for the 15-membered ring complex also apply in these cases. It was found that there is an increase in rate as the ring size increases the respective kH constants being 0.049 dm6 mol-2 s 1 (15-membered), 4.85 dm6 mol 2 s 1 (16-membered) and 1.18 x 103 dm6 

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Asano K, Goeringer D and McLuckey S 1998 Dissociation kinetics in the quadrupole ion trap Proc. 46th Conf Am. Soc. Mass Spectrom. 

Gorse C and Capitelli M 1996 Non-equilibrium vibrational, electronic and dissociation kinetics in molecular plasmas and their coupling with the electron energy distribution function NATO ASI Series C 482 437-49... 

Direct quantitation of receptor concentrations and dmg—receptor interactions is possible by a variety of techniques, including fluorescence, nmr, and radioligand binding. The last is particularly versatile and has been appHed both to sophisticated receptor quantitation and to dmg screening and discovery protocols (50,51). The use of high specific activity, frequendy pH]- or p lj-labeled, dmgs bound to cmde or purified cellular materials, to whole cells, or to tissue shces, permits the determination not only of dmg—receptor saturation curves, but also of the receptor number, dmg affinity, and association and dissociation kinetics either direcdy or by competition. Complete theoretical and experimental details are available (50,51). 

Eqs. (1,4,5) show that to determine the equilibrium properties of an adsorbate and also the adsorption-desorption and dissociation kinetics under quasi-equilibrium conditions we need to calculate the chemical potential as a function of coverage and temperature. We illustrate this by considering a single-component adsorbate. The case of dissociative equilibrium with both atoms and molecules present on the surface has recently been given elsewhere [11]. 

Homolytic bond dissociation, the breaking of a covalent bond with two radicals resulting, generally occurs without any extra activation barrier (see also Chapter 3, Problem 1). Thus, the rate of bond dissociation (kinetics) is directly related to the stabilities of the resulting radicals (thermodynamics). 

Adtveoitages. The antibody technology is remarkably convenient. It requires only that a significant fraction of FLPEP be receptor bound ( 10%). The assay can be set up in a few minutes and is applicable to membranes, permeabilized cells, cells, or any other preparation in which the receptor concentration is in the range of 1 niV or greater. We use this assay routinely to analyze ligand dissociation kinetics. 

From the examples given above it is clear that the development of a novel approach for real time measurements of the dissociation kinetics of reaction intermediates would greatly assist the unravelling of complex multistep processes associated with the transformation of even simple molecules at transition metal centers. While techniques are available for such measurements over a limited range of times, none of the methods are sufficiently general to be useful for extensive measurements. 

Atomic metal ion-hydrocarbon reactions bond dissociation energies for fragments, 15,16t endothermic reactions, 13,15,17f Atomic transition metal ion reactions development of approach for real-time measurements of dissociation kinetics, 39 ion beam apparatus, 12,14f studies of... 

The dissociation kinetics of the nickel chloride complex of the 15-mem-bered, S2N2-macrocycle of type (279) has also been investigated in the presence of hydrochloric acid (Lindoy Smith, 1981). This complex has a similar trans-octahedral structure to that of the 02N2-donor systems just discussed. For the sulfur-containing complex, two consecutive (acid-independent) first-order steps were observed, with the second being slower than the first. The data are in accordance with the scheme ... 

A simple method to analyze binding equilibria with slow association and dissociation kinetics is the precipitation of one binding partner with an antibody or... 

The dissociation of 7 (Scheme 5) from poly[(G-C)] showed three relaxation times and the amplitude corresponded to the total signal, while the dissociation of 7 from poly[(A-T)] was faster and only two relaxation times, corresponding to 70% of the total signal were observed in the stopped-flow experiment. The biological activity of this class of molecules was correlated to the presence of four relaxation times when the dissociation process is measured with DNA, in particular the presence of the long-lived component of hundreds of milliseconds.86,104 105,132 143 The difference in the dissociation kinetics observed with the two polydeoxynucleotides indicates that intercalation into G-C sites is responsible for the biological activity. The dissociation of 7 from ct-DNA led to four relaxation times, a result that is in line with the relaxation times observed with poly[(G-C)] and poly[(A-T)]. 

The effect of structural modification to 22 on the binding efficiency and binding dynamics of hairpin DNA immobilized on metal surfaces were studied using surface plasmon resonance.124,125 The association and dissociation kinetics were analyzed using a sequential model for the binding of the imidazole containing polyamides... 

Formation and dissociation kinetics of nine Ni(II)-macrocyclic tetra-thiaether complexes (eight macrocyclic, one linear in acetonitrile) have been compared with those for Cu(II) analogues and for Ni(II) complexes with macrocylic tetramines (262). Whereas for the tetramine complexes conformational changes may be apparent in the kinetics this is not the case for the tetrathiaether complexes, where there is no kinetic evidence for slow conformational changes after initial bonding of the ligand to... 

UV/Visible Photodissociation. In the case of UV/visible PD the photon energies are high enough to enable direct fragmentation. Previously, UV photodissociation was utilized mainly to study dissociation kinetics of gas-phase ions, but recently it was demonstrated that UV PD could be used for tandem MS of MALDI generated protonated peptides [18]. 

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